1. Field of the Invention
The present invention relates to a projection optical apparatus and more particularly a system for measurement in order to attain the precise alignment between a mask and a substrate.
1. Related Background Art
In the projection optical apparatus of the type in which a pattern on a mask or a reticle is focused on a substrate such as a semiconductor wafer, in order to attain positional alignment between the mask or the reticle and the substrate, the alignment step using alignment marks has been used. That is, when the pattern on the reticle is focused on the substrate while the alignment mark on the reticle is maintained in alignment with the alignment mark on the substrate at a higher degree of accuracy through a projection optical system, the projected pattern image can be formed on the substrate without causing any positional deviation.
However, in general, the projection optical system have distortion so that even when the precise position alignment by using the alignment marks, the relative positional deviation between the projected image and the substrate results due to distortions in the projection optical system.
A degree of such positional misalignment due to distortions described above depends upon the height of the image. Therefore it has been desired that in order to attain the precise alignment between the projected image and the substrate, the distortion characteristics of the projection optical system must be measured so that the attained positional alignment must be corrected by using the alignment marks.
Especially in the case of the stepper, that is, an exposure apparatus of the type in which the pattern on the reticle is focused repeatedly at a reduced scale over a plurality of regions of a substrate so that a plurality of chips are defined on the substrate, if the direction and the quantity of distortion corresponding to the position of a chip can be evaluated at a high dggree of accuracy, a suitable correction of the positional alignment between each chip and the projected image can be accomplished.
Meanwhile, in the case of the exposure apparatus in which the exposure light used to focus an image and the alignment light are different in wavelength from each other, chromatic aberrations of both the exposure light and the alignment light are corrected in the projection optical system. In this case, for example, a correction optical system for varying the optical length of the a1ignment light is incorporated.
The distortion quantity in the projection optical system can be divided into a distortion in the S (sagittal) direction and a distortion in the M (meridional) direction in order to evaluate the distortion quantity.
In the case of the projection optical system in which chromatic aberrations for two light waves are corrected as described above, it is in general difficult to make the distortion quantities of the two light waves same in the all directions. As a result, there arises the problem that even when the precise alignment can be precisely attained, there is a possibility that the position of the projected image is different from an optimum exposure position in practice.
Furthermore, even in the case of the correction in response to the simultaneous measurements of the magnifications (the size of the projected image of the reticle pattern) in the projection optical system, there arises the problem that a magnification offset (errors in measurement) results.